Cell a Basic Structural and Functional Unit of Life

WHAT IS A CELL?

We know that the body of all living organisms is made up of cells which

carryout certain basic functions. Hence the cells are called “Basic structural and functional units of living organisms”. The classical branch of biology that deals with the study of structure, function and life history of a cell is called

“Cell Biology”

Robert Hooke (1665): He is an English scientist who observed honeycomb like dead cells in a thin slice of cork under microscope. He coined the term ‘cell ‘, which means a small room or compartment

Anton Von Leeuwenhoek (1667): First saw and described living cell

Matthias J Schleiden(1838):a German botanist based on his studies in different plant cells and Theodore Schwann (1839 ), a British zoologist based on his studies on different animal cells formulated ’cell theory’ 

CELL THEORY:

Cell theory was formulated by M J Schleiden (1838) and Theodore Schwann (1839 ). The main principles of cell theory are

• All living organisms are composed of cells and products of cells
• All cells arise from pre existing cells through the process of cell division
• The body of living organisms is made up of one or more cells 

ORGANISMS SHOW VARIETY IN CELL NUMBER, SHAPE AND SIZE

The invention of electron microscope and staining techniques helped scientists to study the detailed structure of cell. The number of cells vary from a single cell to many cells in an

organism. The organisms made up of a single cell are called unicellular organisms. These are capable of independent existence. The single cell carries all the functions like digestion, excretion, respiration, growth and reproduction. So, they are rightly called a cellular organisms

Eg : Amoeba, Euglena, Paramecium etc.

The organisms made up of more than one cell are called multicellular organisms. In multicellular organisms the cells vary in their shape and size depending on their function. The cells are spherical, oval, polyhedral, discoidal, spindle shaped, cylindrical in shape. The shape

of the cells varies with the functions they perform. 

Eg:   Parenchyma cells – Polyhedral cells that perform storage function

         Sclerenchyma cells – Spindle shaped cells that provide mechanical support

         White blood cells – Amoeboid cells that defend the body against pathogens

         Nerve cells – Long and branched that conduct nerve impulses

         Muscle cells – cylindrical or spindle shaped cells concerned with the movement of body

parts 

The size of the cell varies from few micrometers (μm) to few centimeters (cm). The size of bacteria varies from 0.1 to 0.5 μm. The smallest cell PPLO (Pleuro pneumonia like organism) is about 0.1 μ in diameter. The largest cell is an ostrich egg that measures 170 to 180 mm in diameter. Some Sclerenchyma fibres measure up to 60 cm in length. However the average size of the cell ranges from 0.5 to 10 μm in diameter. 

CELL STRUCTURE AND FUNCTION:

A typical cell has an outer non living layer called cell wall. The cell membrane is present below the cell wall. The cell membrane encloses protoplasm. The protoplasm has a semi fluid matrix called cytoplasm and a large membrane bound structure called Nucleus. The cytoplasm has many membrane bound structures like endoplasmic reticulum, golgibodies, mitochondria, plastids, micro bodies, vacuoles; and non membranous structures like Centrosome and ribosomes. These are called cell organelles. The cytoplasm without these cell organelles is called cytosol. The cytoplasm also contains non living inclusions called ergastic substances and cytoskeleton (microfilaments and microtubules)

ANIMAL CELL

PLANT CELL

Comparison of plant and Animal cell
Plant cell		Animal Cell
Cell wall is present		Cell wall is absent
Centrioles are absent		Centrioles are present
Plastids are present		Plastids are absent
Have large vacuole		May have small vacuoles